1. Field of the Invention
The present invention relates generally to electromagnetic coils for Magnetic Resonance (MR) Systems and more particularly, to electromagnetic gradient coils having a higher inductance value for increasing performance and decreasing power consumption of existing MR resonant gradient power supplies.
2. Description of the Prior Art
A MR system consists of several major component subsystems, which include a main magnet, a gradient subsystem, a radio frequency (RF) reception and transmission subsystem, and a computer control system. A factor of considerable performance in Magnetic Resonance (MR) system performance is not the field strength produced by the main magnet, but the performance of the gradient subsystem. The field strength does however determine the frequency at which the system operates at and the signal to noise ratio of the MR signal. The introduction of MR systems in the mid-1970's resulted in an initial flourishing of electromagnetic coil design for the production of the main magnetic field. The most significant step in this field was the development of super-conducting technology that in a matter of ten years, raised the attainable field strength from 0.1 Tesla (T) to systems with fields over 1 T. The last ten years has seen the standard field strength rise to 1.5 T (15.times.the field of the first human MR imaging system at the University of Nottingham, UK), and the maximum field strength rise to 4.25 Tesla (System currently operational in the Columbia University). Today the field strengths continue to rise at a diminishing rate and the advantages, such as increased signal to noise ratio, rise commensurably slowly. Though, additional benefits, such as the ability to perform functional imaging, also accrue from utilizing higher magnetic fields.
During the last ten years the performance of MR systems has continued to increase dramatically, due to the improvements in the gradient subsystem, the development of new and improved techniques (many of which are highly dependent on the performance of the gradient subsystem, such as high speed imaging), and the technology in general (RF coil design, and computing power). The most significant of these developments has been related to the gradient subsystem, which determines the limitations of MR techniques, such as the resolution and the high speed imaging capability. Consequently, to a large degree, the gradient subsystem determines the diagnostic limitations of the MR system.
Major advances in two components of the gradient subsystem are responsible for this increased performance. First, the output of the power supplies contained in the gradient sub-system has increased considerably. Secondly, gradient coil design methods have also greatly enhanced MR imaging and spectroscopy. One of these new design techniques involves distributed designs for magnetic shielding, which eliminates the external field that would otherwise give rise to undesirable eddy currents and consequent field perturbations. This type of shielding is exemplified by U.S. Pat. No. 4,978,920 to Mansfield et al., entitled MAGNETIC FIELD SCREENS, issued on Dec. 18, 1990. Another new design technique involves providing magnetic fields of better linearity over larger fractions of the interior space, which is exemplified by U.S. Pat. No. 5,266,913 to Chapman, entitled SCREENED ELECTROMAGNETIC COIL OF RESTRICTED LENGTH HAVING OPTIMIZED FIELD AND METHOD, issued on Nov. 30, 1993. More recently acoustic shielding has been introduced, which reduces the disturbing amounts of acoustic noise associated with Lorentz forces generated by the rapid switching of high currents in the presence of large magnetic fields.
From the perspective of the diagnostic MR, and particularly MR imaging techniques, the important factors of gradient performance includes short spin and gradient echo times, high resolution, fast data acquisition rates and good spatial localization. All of these parameters rely on spatial encoding of MR signals. This is achieved by differentially evolving the phase of nuclear spins in different regions of the magnetic field. In a gradient field G(t) which varies linearly in the spatial direction S, The spin phase evolution .psi.(t) is given by: EQU .psi.(t)=.gamma..intg.G(t)dt (1)
where .gamma. is the gyromagnetic ratio of the nuclei under examination. I.e., the relative evolution at any point in the field is proportional to the total integrated gradient field at that point. Thus, the performance of the gradient sub-system in terms of the amount of spin evolution per unit time it can produce is determined by the strength of the gradient field produced and the rate at which this field can be switched on and off (the rise time). The higher the gradient field and the shorter the rise time, the better the performance of the gradient sub-system.
The gradient sub-system itself is composed of two distinct components, the power supply and electromagnetic gradient coils. The performance of the gradient sub-system is determined by the limitations of the power supplies and the efficiency of the gradient coils. The requirement of short rise times conventionally requires gradient coils with minimum inductance values, which are activated with the highest voltage available. Designing such minimum inductance gradient coils has been discussed in an article by Robert Turner, entitled MINIMUM INDUCTANCE COILS, published in J. Phys. E.: Sci. Instrum., Vol. 21, Pp. 948-953, 1988. The requirement for a relatively low inductance to accommodate the short rise time, results in coils that require large currents to provide a large gradient field strength. The simultaneous requirements for high voltage and high current must be provided by the power supply.
At present there are two approaches to satisfying these high power demands. The first, employs a bank of power supplies to provide a sufficiently high current to maintain the gradient field, and a sufficiently high voltage to overcome the inherent coil inductance so as to produce a rise time that is acceptably short. The second approach employs an additional resonant circuit incorporating the gradient coil as the inductive component. The power supply is thereby relieved of the task of totally energizing the magnetic field and has only to replenish the resistive losses.
Presently, the power requirements on existing resonant gradient systems are such that the values of the electrical components are fixed. Consequently, this fixes the period of the resonant gradients. Although, resonant gradient power supplies provide higher gradient strengths and shorter rise times than conventional power supplies, such supplies do not provide a great deal of flexibility. Thus, resonant gradient power supplies are presently restricted for use in high speed Echo Planar Imaging (EPI).
The lack of flexibility of the gradient power supply is one of the main technological limitations of present MR systems. Presently, existing MR systems are upgraded by purchasing additional power supplies and/or replacing the gradient coils.
By designing gradient coils with a higher inductance specifically for use with resonant power supplies, gradient performance can be greatly improved, power requirements reduced considerably and flexibility restored. This in turn will improve the performance of MR systems. In particular, by purposefully manufacturing resonant electromagnetic gradient coils in the proposed manner, the power requirements for the MR system are substantially reduced (&lt;&lt;50%) over the present technology, and considerably higher field strengths can be achieved in shorter times.
It is therefore, an object of the present invention to provide a gradient coil having a substantially higher inductance to increase performance and decrease power consumption in existing resonant gradient power supplies utilized in Magnetic Resonance Systems.